1. Field of the Invention
The present invention relates to germanium-silicon oxide, germanium-silicon oxynitride and silica-germania-titania materials containing large cation fractions of germanium, processes to deposit and densify these materials, and more particularly to such materials that are suitable for use in optical waveguides.
2. Technical Background
Planar optical devices are comprised of a waveguide core embedded in a waveguide cladding material, wherein the waveguide core has an index of refraction that is higher than the index of refraction of the cladding. The device is generally supported by a substrate. The waveguide core and cladding materials are typically either a silica-based glass or a polymer. Silica glass materials can be deposited by flame hydrolysis deposition (FHD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), and physical vapor deposition, including sputtering, electron beam deposition, etc. Flame hydrolysis deposition and plasma enhanced chemical vapor deposition are preferred techniques for depositing silica-based glass waveguide materials (including cladding and core) because these techniques allow higher production rates and the resulting waveguides exhibit low propagation losses. Because planar optical devices are typically designed for use with silica optical telecommunications fiber, the refractive index (n) of the waveguide materials of the planar optical device is typically matched to that of the fiber (usually n is about 1.44 at 1550 nm). The dimensions of the waveguide core are typically about 8 xcexcm in width, and the refractive index of the core typically exceeds the refractive index of the cladding by between about 0.2% and about 2%. This refractive index difference between a waveguide core and cladding is generally achieved by doping the core material with nitrogen or a higher refractive index oxide such as germania, phosphorous trioxide, titania, zirconia, etc. in order to raise its refractive index above that of the cladding. Alternatively, dopants such as boron and fluorine may be added to the cladding to depress its refractive index below that of the core.
The fabrication of a planar optical cross-connect signal switching device based on liquid crystal cells may be performed using prismatic liquid crystal cells and a planar waveguide circuit that matches the ordinary refractive index of the liquid crystal. For a typical liquid crystal (e.g., BL009, described herein) the ordinary refractive index at 1550 nanometers is on the order of 1.501. Thus, a preferred planar waveguide circuit for use in a liquid crystal-based cross-connect device has a refractive index at 1550 nm of about 1.501. Use of a typical planar light wave circuit with a refractive index matched to silica (n=1.44 at 1550 nm) leads to degradation of the optical performance of the cross-connect (i.e., increased optical cross-talk).
Because liquid crystal based optical cross-connect switching devices requires a ground electrode for applying an electrical field across the liquid crystal-filled trench in order to alter the orientation of the liquid crystal molecules to achieve switching, the use of a silicon substrate upon which the ground electrode may be deposited or a single crystal silicon substrate doped to be conductive is highly preferred when fabricating such devices. In order to prevent large thermal stresses which can cause strain-induced birefringence and excessive wafer curvature [which complicates photolithography and alignment of input and output optical fibers (pigtails)], it is important that the waveguide material, in addition to matching the ordinary refractive index of the liquid crystal (n1550xcx9c1.501), also have a coefficient of thermal expansion that matches that of the silicon substrate.
Silicon oxynitride glass materials having a refractive index of about 1.5 at 1550 nm may be made by adjusting the nitrogen content of the glass. However, a SiON film with a refractive index of about 1.5 at 1550 nm would have a coefficient of thermal expansion of about 1xc3x9710xe2x88x926xc2x0 C.xe2x88x921, as compared with a coefficient of thermal expansion of about 3.8xc3x9710xe2x88x926xc2x0 C.xe2x88x921 for a silicon substrate. The low coefficient of thermal expansion (relative to silicon) of the SiON film having a refractive index of 1.5 at 1550 nm would consequently result in a large film strain and wafer curvature. In addition, adding nitrogen to the glass also increases the concentration of Nxe2x80x94H bonds. The Nxe2x80x94H bond has an overtone at 1510 nm which causes strong optical absorption in the 1550 nm communication band.
Thus, in order to fabricate planar optical cross-connect optical signal switching devices based on liquid crystal cells, there is a need for optical waveguide materials having a refractive index of about 1.5 at 1550 nm (i.e., matched to the ordinary refractive index of the liquid crystal) and a coefficient of thermal expansion of from about 3xc3x9710xe2x88x926xc2x0 C.xe2x88x921 to about 4.4xc3x9710xe2x88x926xc2x0 C.xe2x88x921 (i.e., matched to a silicon substrate).
This invention provides glass waveguide materials having a refractive index of about 1.5, and a coefficient of thermal expansion of from about 3xc3x9710xe2x88x926xc2x0 C.xe2x88x921 to about 4.4xc3x9710xe2x88x926xc2x0 C.xe2x88x921. Because these materials have a refractive index closely matched to the ordinary refractive index of a typical liquid crystal material, and a coefficient of thermal expansion closely matched to that of a silicon substrate, the materials of this invention are extremely useful for the fabrication of waveguides for use in liquid crystal based cross-connect optical switching devices.
In accordance with one preferred aspect of this invention, a germanium-silicon oxide or germanium-silicon oxynitride glass composition having a Ge/(Si+Ge) mole ratio of from about 0.25 to about 0.47 and an N/(N+O) mole ratio of about 0.1 or less is used in a liquid crystal based cross-connect optical switching device. The germanium-silicon oxide or oxynitride glass compositions of this invention preferably have a refractive index of from about 1.48 to about 1.52 at 1550 nm and a coefficient of thermal expansion at room temperature of from about 3xc3x9710xe2x88x926xc2x0 C.xe2x88x921 to about 4.4xc3x9710xe2x88x926xc2x0 C.xe2x88x921.
Additionally this invention provides a process to densify films of germanium-silicon oxide glass compositions having a Ge/(Si+Ge) ratio from about 0.25 to about 0.47, and a process to deposit fully dense films of germanium silicon oxynitride glass which have a composition having a Ge/(Si+Ge) ratio of from about 0.25 to about 0.47 and an N/(N+O) ratio of about 0.1 or less.
In accordance with another preferred aspect of this invention, a silica-germania-titania glass composition having a Ge/(Si+Ge+Ti) mole ratio of from about 0.08 to about 0.17 and a Ti/(Si+Ge+Ti) mole ratio of from zero to 0.08 is provided for use in a liquid crystal based cross-connect optical switching device. The silica-germania-titania glass compositions of the invention preferably have a refractive index of from about 1.48 to about 1.52 at 1550 nm, and a coefficient of thermal expansion at room temperature of from about 3xc3x9710xe2x88x926xc2x0 C.xe2x88x921 to about 4.4xc3x9710xe2x88x926xc2x0 C.xe2x88x921.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.